1. Field of the Invention
The invention pertains to a method of preparing synthetic rutile from a titaniferous slag containing magnesium values by a two-step method comprising contacting the slag with chlorine at a high temperature, and subsequently contacting the chlorine-treated slag with hydrochloric acid at an elevated temperature.
2. Description of the Prior Art
Titanium dioxide is a white pigment widely used in the paint, paper and plastic industries. Presently this pigment is manufactured by either one of two processes, the classic sulfate process or the relatively new chloride process. Both processes are described in some detail in Kirk-Othmer's Encyclopedia of Chemical Technology. 3rd Edition, Vol. 23 at pp. 143-148. Although the sulfate process is the dominant source of titanium pigment today, the chloride process is growing more rapidly because it is more energy-efficient and less environmentally difficult. However, the feedstock requirements for the chloride process are more demanding, generally requiring rutile-grade material for operation.
Rutile is a naturally occurring mineral which contains 90 or more percent titanium dioxide, and it can be used as a feedstock in the chloride process with little, if any, upgrading. Unfortunately, rutile is in relatively scarce supply.
High grade ilmenite beach sand also is a naturally occurring mineral (containing 60 or more percent titanium dioxide), and it, too, can be used as a feedstock in the chloride process with little, if any, upgrading. While more abundant than rutile, this mineral unfortunately also is in relatively scarce supply.
Medium-grade (50-55 percent titanium dioxide) and low-grade (less than 50 percent titanium dioxide) ilmenite beach sands are considerably more abundant than either rutile or high-grade ilmenite beach sand, but neither can be used in the chloride process without being upgraded. Upgrading usually is accomplished by one of two processes, i.e. thermo-reduction followed by acid-leaching, or electro-smelting. Both of these processes reduce the amount of iron and other impurities in the ilmenite beach sand.
Rock ilmenite (37-45 percent titanium dioxide) is the most abundant source of naturally occurring titanium dioxide, but it cannot be used as a feedstock for the production of titanium pigment without first being upgraded. Rock ilmenite usually is upgraded by electro-smelting which is effective for removing the iron values and producing a concentrate known as a titaniferous slag. The titanium values present in the slag principally are in the form of titanium dioxide (TiO.sub.2) and titanium sesquioxide (Ti.sub.2 O.sub.3), and the iron values principally are in the form of ferrous oxide (FeO) and metallic iron (Fe.degree.). While electro-smelting removes enough iron values from the rock ilmenite to render the slag suitable as a feedstock for the sulfate process, it usually does not remove enough magnesium and calcium values to render the slag suitable as a feedstock for the chloride process. Consequently, only that rock ilmenite naturally low in alkaline earth metal values, particularly magnesium, will produce slags suitable as a feedstock for the chloride process. Slags produced from rock ilmenites naturally high in alkaline earth metal values are generally only suitable as feedstocks for the sulfate process unless further upgraded to reduce the alkaline earth metal content.
As is well known, the chloride process is a fluidized-bed process and the presence of too much magnesium, like too much iron, will promote the formation of paste-like condensates of magnesium chloride which eventually will clog the reaction bed, conduits, valves and other elements of the equipment. Consequently, abundant rock ilmenite is not available as a source of feedstock for the chloride process unless it either is naturally low in magnesium or the magnesium content is lowered through processing.
Daubenspeck and McNeil teach in U.S. Pat. No. 2,747,987 a process for selectively chlorinating a slag containing reduced titanium values by first reducing the slag to particulate size, and then contacting it with chlorine gas in a static or moving bed operation at a temperature between 550.degree. and 950.degree. C. The chlorine reacts with the iron oxide in the slag to produce volatile ferric chloride, and thus reduces the iron content of the slag. Daubenspeck and McNeil neither discuss the need for, nor a method of, reducing the magnesium values in the slag. In addition, the process is not autogenous and preheating the feedstock in the substantial absence of free oxygen is necessary (as disclosed by Gueguin in U.S. Pat. No. 4,629,607). This process is best applied to low magnesium oxide-containing slags since relatively small amounts of that impurity actually are chlorinated.
The titanium values in the foregoing products contain a significant portion of the titanium as reduced titanium. As here used, the terms "reduced titanium" and "reduced titanium values" mean low valent titanium values and are definitive of titanium compounds and complex compositions in which the titanium values are present in the trivalent or divalent state.